The invention relates generally to inspection of wall-flow particulate filters, and particularly to a method and apparatus for detecting defects in a wall-flow particulate filter.
Solid particulates in fluids such as exhaust gas are typically removed using wall-flow particulate filters having a honeycomb structure. FIG. 1 illustrates a typical wall-flow, honeycomb, particulate filter 100. The honeycomb filter 100 has an inlet end face 102 and an outlet end face 104 and an array of porous walls 106 extend longitudinally from the inlet end face 102 to the outlet end face 104. The porous walls 106 intersect each other to define a grid of generally parallel inlet cells 108 and outlet cells 110. The outlet cells 110 are preferably closed with plugs 112 where they adjoin the inlet end face 102 and open where they adjoin the outlet end face 104. Similarly, the inlet cells 108 are preferably closed with plugs (not shown) where they adjoin the outlet end face 104 and open where they adjoin the inlet end face 102. In a typical cell structure, each inlet cell 108 is bordered on one or more sides by outlet cells 110 and vice versa, preferably in a checkerboard pattern. The inlet and outlet cells 108, 110 may have a square cross-section as shown in FIG. 1 or may have other cell geometry, e.g., rectangle, triangle or hexagon.
In operation, the honeycomb filter 100 is installed in an appropriate can and inserted into the exhaust system of a vehicle equipped with a diesel engine. During operation of the vehicle, diesel exhaust is directed at the inlet end face 102 of the honeycomb filter 100. The diesel exhaust flows into the inlet cells 108 of the honeycomb filter 100, passes through the porous walls 106 into the outlet cells 110, and exits the filter at the outlet end face 104. The porous walls 106 retain a desired portion of the solid particulates in the exhaust.
Filtration efficiencies up to and in excess of 90% by weight of the diesel exhaust particulates can be achieved with honeycomb filters such honeycomb filters 100. However, the filtration efficiency achievable can be dramatically reduced if there are leaks in the honeycomb filter. Such leaks may be due to straight-through defects (cracks, tears, large pores) in the interior porous walls and/or defects in the plugs of the filter (defects include missing plugs). Such defects allow the exhaust gas to pass through the interior walls and/or plugs of the filter uninhibitedly. It is desirable to find such defects and repair them, where possible, prior to using the honeycomb filter in particulate filtration applications. In cases where the honeycomb filters are made by extrusion from ceramic materials, such as cordierite, aluminum titanate and silicon carbide, the defects may be detected before or after firing of the honeycomb body. In the former case, the interior walls are nonporous. In the latter case, the interior walls are porous. Typically, it is easier to repair defects while the honeycomb body is unfired or green.
From the foregoing, it should be apparent there is a need for an improved method of detecting straight-through defects in honeycomb filters which reduce inspection time and improve detection reliability.